Jewish Calendar

Level: Basic

A few years ago, I was in a synagogue, and
I overheard one man ask another, "When is
Chanukkah this year?" The other man smiled
slyly and replied, "Same as always: the 25th of Kislev." This humorous comment
makes an important point: the date of Jewish
holidays does not change from year to year. Holidays are celebrated on
the same day of the Jewish calendar every year, but the Jewish year is not
the same length as a solar year on the Gregorian calendar used by most of
the western world, so the date shifts on the Gregorian calendar.

The Jewish calendar is primarily lunar, with each month beginning on the
new moon, when the first sliver of moon becomes visible after the dark of
the moon. In ancient times, the new months used to be determined by observation.
When people observed the new moon, they would notify the Sanhedrin. When
the Sanhedrin heard testimony from two independent, reliable eyewitnesses
that the new moon occured on a certain date, they would declare the rosh
chodesh (first of the month) and send out messengers to tell people when
the month began.

The problem with strictly lunar calendars is that there are approximately
12.4 lunar months in every solar year, so a 12-month lunar calendar loses
about 11 days every year and a 13-month lunar gains about 19 days every year.
The months on such a calendar "drift" relative to the solar year. On a 12
month calendar, the month of Nissan, which is supposed to occur in the Spring,
occurs 11 days earlier each year, eventually occurring in the Winter, the
Fall, the Summer, and then the Spring again. To compensate for this drift,
an extra month was occasionally added: a second month of Adar. The month
of Nissan would occur 11 days earlier for two or three years, and then would
jump forward 29 or 30 days, balancing out the drift.

In the fourth century, Hillel II established a fixed calendar based on
mathematical and astronomical calculations. This calendar, still in use,
standardized the length of months and the addition of months over the course
of a 19 year cycle, so that the lunar calendar realigns with the solar years.
Adar II is added in the 3rd, 6th, 8th, 11th, 14th, 17th and 19th years of
the cycle. The current cycle began in Jewish year 5758 (the year that began
October 2, 1997).

In addition, Yom Kippur should not fall
adjacent to Shabbat, because this would cause
difficulties in coordinating the fast with Shabbat, and
Hoshanah Rabba should not fall on Saturday
because it would interfere with the holiday's observances. A day is added
to the month of Cheshvan or subtracted from the month of Kislev of the previous
year to prevent these things from happening.

The year number on the Jewish calendar represents the number of years since
creation, calculated by adding up the ages of people in the Bible back to
the time of creation. However, this does not necessarily mean that the universe
has existed for only 5600 years as we understand years. Many
Orthodox Jews will readily acknowledge that
the first six "days" of creation are not necessarily 24-hour days (indeed,
a 24-hour day would be meaningless until the creation of the sun on the fourth
"day"). For a fascinating (albeit somewhat defensive) article by a nuclear
physicist showing how Einstein's Theory of Relativity sheds light on the
correspondence between the Torah's age of the
universe and the age ascertained by science, see
The
Age of the Universe.

Jews do not generally use the words "A.D." and "B.C." to refer to the years
on the Gregorian calendar. "A.D." means "the year of our L-rd," and we do
not believe Jesus is the L-rd. Instead, we use the abbreviations C.E. (Common
or Christian Era) and B.C.E. (Before the Common Era).

The "first month" of the Jewish calendar is the month of Nissan, in the spring,
when Passover occurs. However, the
Jewish New Year is in Tishri, the seventh month,
and that is when the year number is increased. This concept of different
starting points for a year is not as strange as it might seem at first glance.
The American "new year" starts in January, but the new "school year" starts
in September, and many businesses have "fiscal years" that start at various
times of the year. Similarly, the Jewish calendar has different starting
points for different purposes.

The names of the months of the Jewish calendar were adopted during the time
of Ezra, after the return from the Babylonian exile. The names are actually
Babylonian month names, brought back to Israel by the returning exiles. Note
that most of the Bible refers to months by number, not by name.

The Jewish calendar has the following months:

Name

Number

Length

Gregorian Equivalent

Nissan

1

30 days

March-April

Iyar

2

29 days

April-May

Sivan

3

30 days

May-June

Tammuz

4

29 days

June-July

Av

5

30 days

July-August

Elul

6

29 days

August-September

Tishri

7

30 days

September-October

Cheshvan

8

29 or 30 days

October-November

Kislev

9

30 or 29 days

November-December

Tevet

10

29 days

December-January

Shevat

11

30 days

January-February

Adar

12

29 or 30 days

February-March

Adar II

13

29 days

March-April

In leap years, Adar has 30 days. In non-leap years, Adar has 29 days.

The length of Cheshvan and Kislev are determined by complex calculations
involving the time of day of the full moon of the following year's Tishri
and the day of the week that Tishri would occur in the following year. I
won't pretend to understand the mathematics involved, and I don't particularly
recommend trying to figure it out. There are plenty of easily accessible
computer programs that will calculate the Jewish calendar for more than a
millennium to come.

Note that the number of days between Nissan and Tishri is always the same.
Because of this, the time from the first major festival
(Passover in Nissan) to the last major festival
(Sukkot in Tishri) is always the same.

If you would like to download a Jewish calendar for your use, I highly recommend
Calendar Maven's Hebrew Calendar, a shareware program that is available for
download at
http://www.calendarmaven.com.
With this program, you can see calendars for dates from the Gregorian year
1600 to the year 2200, including holidays, weekly Torah readings, candle
lighting times and more.

If you would like to look up the date of a Jewish holiday, from the Gregorian
year 1 to the Gregorian year 9999, try
http://www.hebcal.com. I don't know
how accurate this is (especially given that during the earlier dates, months
were determined by observation), but I haven't caught any mistakes in it
yet.